The present invention relates to a device for monitoring electric power in a plurality of circuits and, more particularly, to a modular string monitor for monitoring DC current and/or voltage in a plurality of conductors.
Photovoltaics (PV) is the generation of electric power by converting solar radiation to direct current electric power with semiconductor material contained in a solar cell. A 100 square centimeter (cm2) silicon solar cell produces about 2 Amps (A) at about 0.5 volts (v) when irradiated by 1000 Watts/ square meter (W/m2) sunlight. To obtain commercially useful electric power, multiple solar cells are electrically connected in series in a solar module or panel. Multiple solar modules are, in turn, connected electrically in series in groups called “strings” and the DC electricity output by a plurality of strings is combined in a combiner. The DC output of one or more combiners of a PV power generation site is transmitted to an inverter for conversion to alternating current (AC) power for powering local AC loads and/or transmission to a power grid.
The operation of a photovoltaic power generation site can be monitored at several levels. The AC output of the inverter(s) of the solar site is typically measured by site level monitoring. Site level monitoring may involve net or bi-directional metering to account for power obtained from the power grid when solar generation is inadequate to satisfy local demand and to account for power transmitted to the grid when excess solar generated power is available. Site level monitoring is relatively inexpensive, requiring only a single meter and addresses the commercial need to account for the power generated or consumed at a solar power site. However, site level monitoring is not particularly useful for monitoring the performance of a site or for identifying and rectifying problems that may arise. There are a variety of factors which can affect the output of a solar generating site and many, like cloud cover, are transient and local. This make it difficult to initially establish an expected output for the site and, when the site is operating, to recognize if a change in output is due to a problem with a portion of the array or one of the many other factors that can affect the site's output. With site level monitoring, a change of less 10% in the site's output may not be recognizable as indicating a problem with a portion of the array. Even if a problem with the array is suspected, troubleshooting typically involves costly selective dismantling and testing of portions of the array by skilled technicians because site level monitoring provides little assistance in localizing the problem.
An array level power meter monitoring the DC output of a combiner provides insight into the operation of the site's array. If a module is damaged or defective, the output of the string of which it is an element will be reduced and, as a result, the output of the combiner to which the string is connected will be reduced. An array level power meter is typically connected to a site control and monitoring system and the average output of one array level power meter is commonly used as a reference for other array level meters of the site. If the current or voltage output sensed by one of the array level meters varies from the output of the reference meter, the control system will alert the site operator and troubleshooting and repair can be initiated. While array level monitoring requires a monitor for each combiner of the site, site operation benefits from earlier problem detection and from more rapid and less costly identification of an underperforming portion of the array.
Monitoring the current, voltage or power of each string of an array enables rapid recognition of problems in the array by enabling comparison of the performance of a string with the performance of its neighbors or the string's theoretical performance calculated from measurement of the incident solar radiation and ambient temperature. In addition, the cost of troubleshooting, cleaning and repair can be substantially reduced by the ability to identify an underperforming string. However, string level monitoring can be expensive because at least one transducer is required for each of the many strings making up the site and the transducers for sensing DC current have heretofore been relatively expensive.
DC current can be monitored by a Hall effect device incorporated in a toroidal core of magnetic material. The conductor in which the current is to be measured is passed through the core and current flowing in the conductor produces fluctuations in the magnetic field in the core which are proportional to the current's magnitude. To determine the magnitude of the current in the conductor, the fluctuation of the magnetic field produced by the current is sensed by the Hall effect device and compared to a baseline magnetic field established when the transducer was calibrated. Hall effect DC current transducers are expensive and a current surge in the conductor can produce a magnetic field which can permanently alter the core so that the baseline is no longer accurate. Unless the device is recalibrated, any measurements made subsequent to the power surge will also be inaccurate.
DC current is also measurable with a fluxgate magnetometer typically comprising a “sense” or secondary coil surrounding an inner “drive” or primary coil that is wound around a magnetically permeable core. An alternating current is applied to the drive coil which alternately drives the core into plus or minus saturation. The constantly changing magnetic field induces an electric current in the sense coil which is measured by a transducer. When the core is exposed to an external magnetic field produced by an energized conductor, the core will be more easily saturated in alignment with the external field and less easily saturated in opposition to the external field and the difference in the strength of the magnetic field will be related to the magnitude of the current in the conductor. Fluxgate sensors and string monitors comprising multiple fluxgate sensors are also expensive.
What is desired, therefore, is a robust and less expensive string monitor for monitoring DC current and/or voltage in a plurality of conductors.